With the ever increasing global population projected to touch 9.7 billion by 2050, combined with the reducing arable land area, crop productivity has become an important concern. Plants are continuously under the attack of wide range of pathogens namely fungi, bacteria and viruses. These pathogens have numerous ways of adversely affecting the growth of development of plants some of which include (a) expressing a multitude of degradative enzymes like cellulases, hemicellulases, pectinases to destabilize the plant structural framework and facilitate its invasion (Walton,1994) (b) secreting toxins which adversely affects the metabolic homeostasis in plants by inhibiting enzymes and modulating membrane permeability (Quiggly and Gross,1994) (c) promoting hormonal imbalance in the plant leading to abnormal growth and development (Suckstorff and Berg,2003).
Now, plants over the course of evolution have developed two-tier defense against these pathogen attacks. The first constitutive layer is characterized by readymade structures and compounds synthesized during the normal development of plant and the second inducible layer being triggered by pathogen infection. This inducible defence mechanism again branches into PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI).Plants consists of pathogen recognition receptors on the cell surface which on encountering the slowly evolving but essential for survival pathogen-associated molecular patterns (PAMPs), trigger the PTI. PTI involves the activation of multiple processes, like mitogen-activated protein kinase (MAPK) cascades, generation of reactive oxygen species (ROS), altering hormone signaling pathways and expression of defense related genes. But unbeaten pathogens overcome the PTI by secreting virulence related effectors. These effector proteins encoded by the avirulent (avr) genes, on being recognized by intracellular plant receptors encoded by the resistance (R) gene products, initiates an even more robust effector-triggered immunity (ETI) that eventually ends in programmed cell death (Jones and Dangl 2006).
The modus operandi of both these immune responses involve both localized or systematic developments namely, a) rapid production of ROS which not only kills the microbes but also strengthens the plant cell wall against hydrolytic attacks (Lamb and Dixon,1997); (b) build up of antimicrobial secondary metabolites known as phytoalexins (Hammerschmidt 1999); (c) hypersensitive response (HR) leading to cell death thereby further restricting the spread of the pathogen (Hammond-Kosack and Jones 1996; Alvarez et al. 2000). (d) expression of defense related proteins like pathogenesis related (PR) proteins and antimicrobial peptides (AMPs) (Dixon and Harrison,1990).
Besides plants, AMPs have been discovered and characterized from different organisms and have occurred through the course of evolution as an important defense response against pathogen invasion. Study of the natural counterparts coupled with the use of bioinformatics tools has enabled us to generate customized synthetic peptides which are also toxic and effective against the various phytopathogens.
But pathogens with the passage of time have managed to successfully infect the host, inspite of the multilayered defense and lead to crop yield losses. Decades of pesticide usage have a major contribution in protecting against these pathogens, but their heavy usage is responsible for increased environmental issues. In addition, conventional breeding strategies due to lack of sufficient resistance in the germplasm and also being a laborious process have not been successful enough in generating durable resistance. Therefore AMPs due to their direct antimicrobial effect, coupled with genetic engineering have great potential in the development of robust disease resistant crops.
This review will be highlighting the different antimicrobial peptides from both plant and non-plant sources and how they have been utilized in providing resistance in against the different plant pathogens.
Overview of Antimicrobial Peptides
Antimicrobial peptides (AMPs) are small, structurally diverse peptides produced as an integral component of the evolutionarily conserved innate immune system of a wide variety of organisms ranging from insects to humans in order to protect against a broad spectrum of pathogens (Zasloff 2002). They are usually cationic in nature interspersed with hydrophobic residues giving it the ability to interact with the cell membranes of the pathogens. These are an expansive group consisting of peptides and small proteins containing less than 100 amino acids. More than 5500 AMPs of both natural and synthetic origins has made way for the creation of numerous databases, namely
1. PhytAMP: Plant AMP Database (http://phytamp.pfba-lab-tun.org/main.php)
2. CAMP: Collection of Anti-Microbial Peptides (http://www.camp.bicnirrh.res.in)
3. APD: Antimicrobial Peptide Database (http://aps.unmc.edu/AP/main.php)
4. LAMP (http://biotechlab.fudan.edu.cn/database/lamp/index.php)
5. DAMPD: Dragon Antimicrobial Peptide Database (http://apps.sanbi.ac.za/dampd/)
6. RAPD: Recombinantly produced Antimicrobial Peptides Database (http://faculty.ist.unomaha.edu/chen/rapd/index.php)
7. DRAMP: Data Repository of Antimicrobial Peptides (http://dramp.cpu-bioinfor.org/)
8. Defensins knowledgebase (http://defensins.bii.a-star.edu.sg/)
9. YADAMP: Yet Another Database of Antimicrobial Peptides (http://www.yadamp.unisa.it/default.aspx)
10. DBAASP : The Database of Antimicrobial Activity and Structure of Peptides (http://www.biomedicine.org.ge/dbaasp/)
Plant Antimicrobial peptides
The general features of plant AMPs are small molecular size, net positive charge, amphipathic properties, and rich in cysteine residues conferring a high termostability. Recent analyses suggest that plant genomes are rich in genes encoding cysteine-rich peptides resembling AMPs, which might account for up to 2-3% of the predicted genes, suggesting that plant possess a formidable defense arsenal (Silverstein et al. 2007). They are expressed constitutively and also induced by pathogen attack and help in modulating the plant immune response (García-Olmedo et al. 1998). Expression of plant AMPs is also localized to different organs as evidenced by the fact that they been found in leaves, roots, stems, flowers and seeds. The plant AMPs contains 4–12 conserved cysteine residues which stabilize their 3D structure by disulfide bonds. Antimicrobial peptides from plants share similarities in pattern of disulfide bridging and structural characteristics (Odintsova and Egorov 2012) and accordingly have categorized into different classes, namely thionins, defensins, lipid transfer proteins, heveins, snakins, knottin-like, puroindolines and cyclotides .Plant AMPs not only have diverse structures, but also have activity against a wide range of phytopathogens via various mechanisms involving membrane permeabilization, impairment of intracellular process and regulation of the plant immune machinery (Rahnamaeian et al. 2011).
Mode of action of plant antimicrobial peptides
Antimicrobial peptide families possess unique and conserved structures and amino acid composition allow them to distinguish host tissue and selectively act against the pathogens. This selectivity is also a result of the structure and composition of host and pathogen membrane surface. The AMPs on interacting and binding with the surface, affect pathogens in mainly two ways: a) Permeabilization of the cell membrane b) Incapacitating the intracellular machinery
AMPs due to their cationic nature and distinctive structural features enable to interact with the negatively charged lipids of the cell membrane and change the membrane topology. The electrostatic interactions between the peptide and the membrane lipids coupled with the buildup of the AMPs above the threshold level on the membrane surface, sets into motion the collapse of the pathogen membrane (Pelegrini and Franco 2005). This occurrence is explained by three different models:
• Barrel-stave model: The peptides act as monomers and undergo oligomerization upon aggregation on the membrane surface. This leads to alignment of the hydrophobic residues with hydrophobic core of the membrane and polar residues to form inner lining of the pore.
• Toroidal or wormhole model: The peptide intercalates itself into the membrane interior and disrupts the hydrophobic interior by favoring the realignment of the phospholipid heads. The polar head groups curve into and forms the lumen of the pore unlike in the barrel-stave model.
• Carpet model: This non-pore forming mechanism involves ‘carpeting’ of the membrane surface with the peptide molecules. Here the peptides orient themselves via electrostatic interactions and upon reaching threshold levels, upsets the membrane stability. In addition, the peptides can act as ‘detergent’ molecules and interact with membrane lipids to form micelles which further breaks down the membrane eventually causing cell death.
Sometimes in addition to membrane pore formation, antimicrobial peptides have been found to act intracellularly and affect the pathogen. They cross the membrane and act via a wide variety of mechanisms: depolymerization of actin (Koo et al. 2004), binding of the fungal cell wall chitin (Fujimura et al. 2005), generation of ROS and subsequent cell death (Aerts et al. 2007), cell cycle inhibition (Lobo et al. 2007), cytoplasm granulation (van der Weerden et al. 2008), impairment of DNA synthesis (Haney et al. 2013), disruption of cell signaling (Nanni et al. 2014).
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